Human Life History Flashcards
Life History is key characteristics that define progress through life (SG)
-Pattern of growth
-Development
-Maturation
-Reproduction
-Mortality
Why are life histories important? (SG)
-General trends/patterns
o Limitations, development time, energy requirements
-Human life history affects fitness and disease susceptibility
-Affects reproduction and longevity which in turn affects evolution
-Understanding underpinning mechanisms
Understanding 4 underpinning mechanisms for life histories
- Limitations imposed by mechanical strain on bones – if you are bigger its harder to distribute oxygen and nutrients
- Energy allocation – more energy b/c bigger brain, fire= more food=bigger brain
- Longer time required to reach larger size- elephant gestation period 2-3yrs
- Advantage of a larger body size is traded off against the greater risk of death during a longer maturation period
R vs K Reproductive strategies (SG)
R: small, short life span, prey, less intelligent, larger litter, reproduce early, independent early, little care for offspring, small size at birth, high fecundity
K: big, longer lifespan, predator, intelligent, smaller litter, reproduce late, dependent, care for offspring, larger size at birth, low fecundity
fecundity
of offspring and individual will have in their whole lifetime
Single cycle = semelparous breeding
-Reproduce once before dying
-High investment in mating
-Low investment in individual offspring survival
-Large # of offspring
-Ex: salmon
Multiple reproductive events = iteroparous
-High investment in offspring survival
-Long reproductive lifespan
-Trade off = fewer offspring from each mating
-humans
Factors affecting Current vs Future reproduction
-Environmental factors affect life history traits – plasticity
-Famine decreases fecundity
o Anorexia and amenorrhea (don’t get period)
-Early puberty and reduced height gain during puberty in cases of famine, childhood abuse…. Etc
o Mechanism enacted to ensure reproduction (evolutionary reason)
Extrinsic vs Intrinsic factors of mortality (SG)
-Mortality can be a driver of evolution
-Intrinsic (internal) = physiology like disease
-Humans primarily die from intrinsic factors
-Extrinsic (external)= environmental like prey/predator or accidents
-Increases in extrinsic mortality rates = promotes earlier reproduction and smaller body size
-Smaller body size = smaller offspring = so more vulnerable to prey = so higher # of offspring produced to compensate for high mortality rate
Longevity and Body mass (SG)
-Bigger body size = lower extrinsic mortality rates = longer lifespan
-Smaller body size = increased extrinsic mortality rate = shorter lifespan
-Prey = small = high extrinsic mortality = R strategy
-Predator = large = low extrinsic mortality = K strategy
Size vs Timing of Sexual Maturity
- 2 competing factors
-Investing energy into continued growth or reproduction?
-2 competing factors:
1. Adult size favors late reproduction where larger female = larger offspring and larger male = more successful mating
2. Mortality risk favors early reproduction
Fecundity and Lifespan (SG)
-Increased lifespan → increased body size → lower extrinsic mortality rates → lower # of offspring
-inverse correlation b/w lifespan and # of offspring
Longer lifespan and disease
-Natural selection doesn’t operate on disorders that develop after reproduction or if don’t affect reproductive fitness
-Longer lifespan = increased risk for developing late onset disorders (cancer, Alzheimer’s, Parkinson’s….)
Mortality risk influences life history
↑ extrinsic mortality → shorter maturity phase → smaller size and shorter lifespan →higher infant mortality (due to small size) → larger # of offspring (to compensate for high infant mortality rates)
Human Lifespan
-Low rate of extrinsic mortality factors because we have no predators and cultural & social support
-Extrinsic factors: homicide & accidents
-More energy for growth and learning leads to a prolonged reproductive phase
-Longer lifespan as we have evolved
-Increased incidence of late onset diseases b/c we are living way longer than we used to
Body Size (SG)
- is limited by
-Body size is limited by: resources, gravity, cardiovascular system, motility, nutrient availability, … etc.
-Larger you are the longer you survive
-Allometry: body size relationship to different components
brain size in humans and life history (SG)
-Increased body weight = increased brain weight
-Humans have 2.5X what is expected for other primates based on our body size
-Brains have high energy consumption: 2% of body weight, 20% of energy consumption, 15% of blood flow
-Larger brains allow for more social and cultural interactions and achievements
-We have immature brains at birth, so they continue to grow post birth
-Larger brain size is associated with social groups
How do we develop larger brains? – 2 main ways
- Grow brains for longer
- Grow brains faster
(both ways) Results in: 2x as many neurons that form more synapses to perform its functions
Human Life History Stages
-Neonate (birth to 2 months)
-Infancy (2- 36 months)
-Childhood (3-7 years)
-Juvenile (girls = 7-10 and boys = 7-12)
-Puberty and adolescent
-Adulthood (20 to end of childbearing age)
-Senescence (end of childbearing age to death)
Human Life history
-Gestation is 280 days
-High parental investment
-Singleton (1 kid at a time)
-1 woman only has a few kids
-Long post-natal dependence
-High survival rate (in prehistoric times only 50% survived)
-Only homos have a childhood phase because it’s for brain growth
-adolescent phase only found in humans because this is where cognitive/ behavioral/social interactions skills develop
brain during growth
-at 2 months in utero the brain makes up 50% of its body mass
-adults brain makes of 2% of body mass
-brain metabolism decreases (less energy needed) as brain grows bigger
Human infancy stage
-rapid brain and immune development
-goes through many milestones
-after 6 months requires more than breast milk
-maternal milk as primary nutrient source
o stress from limited attachment and sensorimotor experiences
o nutrition required for neural connections
o mother reared vs formula fed chimps have affected growth of white and grey matter in the brain
SIDS (SG for factors)
-western cultures at 2-3 months and during sleep
-factors for SIDS: formula fed, low socioeconomic status, premature or low birth weight, smoke exposure, non-back and lone sleeping, over heating soft bedding and materials
-mismatch infant vulnerabilities and dependencies
-campaign say to keep close contact with safe sleeping space
Human Childhood
-b/w weening and 7
-constant growth: developing immune system & brain, adrenal androgen production causing mid-growth spurt, permanent teeth
-adult assistance needed: cooperative breeding, social and technical skills learned
-at 7 kids may be able to survive on their own
-unique to the Homo group to give time for brain development
stunting (SG)
-sacrificing growth in favor of critical body functions
- long term effects: diminished physical & cognitive development, reduced productive capacity, poor health, increased risk for obesity
- happens early on in growth period and is irreversible
-Recommended actions to help: breastfeeding, scale up prevention, maternal nutrition, community support
Juvenile
-7 to puberty (10-14 F & 12-16 M)
-Fully developed immune system
-Not as dependent on parents
-Feed themselves
-Learning social and economic skills
(SG) Extension of period between weening and maturity
- bad
- good
-Risky because: predation, nutrient needs, disease, parental support lowers
-But we need it for:
1.Female size requirements (for pregnancy)
2.Social and foraging skills
3.Food competition in larger groups
4.Brain development
5.Immune system development
Human Puberty and Adolescence
-Biological maturation: growth acceleration for 2-3 years, derived trait for humans, ends with epiphysial plates closing
-Adolescence is post puberty on the way to adulthood and is unique to humans to give us more time for brain growth and to learn social and communication skills
-Sex differences are in the onset and growth spurt rates where Females are 13-15 and Men 15-17
Human Adult
-Completion of brain complexity
-Reproductive life 15-35 years
-Menopause 45-55: oocytes produced in fetal life decline and die
Human Fecundity Historical pattern
-Having kids used to be a valuable resource (labor)
-Industrial revolution brought: more food, less work, less disease so higher fecundity rates
-Now having children is a burden and contraception had become available
Environment and Reproduction
-Exposure to chemicals in our environment leads to lowering fecundity
-Endocrine disrupting chemicals like BPA can cause: increased risk for obesity/T2DM, decreased fertility & birth weight, increase cancer risk
-BPA: mimics estrogen so disrupts hormonal balance
Senescence
-Menopause is not found in other species maybe because no one lives long enough to get to this stage like humans do
-Menopause causes a decline in bone strength
-Female fertility declines before menopause
-Used to have a lifespan of 35-40 but some evidence suggests 60s-70s
-Decline in fertility could be due to a longer lifespan
-Menopause may provide a fitness advantage: grandmother can help rear other kids
Death and Birth Pattern with the Industrial Revolution
increased longevity + decreases mortality = bigger population
brain growth in humans vs chimps (SG)
Homos have a childhood phase for brains growth and Humans have an adolescence phase to learn social/communication skills while chimps do not have either phase only have infancy, juvenile, and adult phase
Why is breastfeeding important to the developing brain? (SG)
b/c breast milk is the primary source of nutrients and is required for the brain to make neural connections and it affects the growth of white/grey matter in the brain
- it can also help prevent SIDS & stunting
How has an increased brain size affected life histories? (SG)
increased brain size has helped us make tools, become hunter, master fire, form social bonds and cultural developments to create communities to help us better survive
examples of life history affecting disease risk (SG)
- intrinsic mortality- diseases are a main cause
-Longer lifespan = increased risk for developing late onset disorders (cancer, Alzheimer’s, Parkinson’s….) - since there is an extension b/w weening and maturity you are more susceptible to disease because immune system not fully formed
- higher fecundity’s during industrial evolution because there was less disease
effects of industrial revolution (SG)
- more food, less work, less disease so higher fecundity rates
-increased longevity + decreases mortality = bigger population
